Pulverizers are well known for the reduction of the particle size of solid fuel to allow for combustion of the solid fuel in a furnace. A pulverizer employs some combination of impact, attrition and crushing to reduce a solid fuel to a particular particle size. Several types of pulverizer mills can be employed for the pulverization of the solid fuel, for example, coal, to a particulate size appropriate for firing in a furnace. These can include ball-tube mills, impact mills, attrition mills, ball race mills, and ring roll or bowl mills. Most typically, however, bowl mills with integral classification equipment are employed for the pulverization of the solid fuel to allow for transport, drying and direct firing of the pulverized fuel entrained in an air stream.
Bowl mills have a grinding ring carried by a rotating bowl. Fixed position rollers are mounted on roller journal assemblies such that the roll face of the rollers are approximately parallel to the inside surface of the grinding ring and define a very small gap therebetween. Pressure for grinding is applied through springs or hydraulic cylinders on the roller journal to crush solid fuel caught between the roll face of the roller and the grinding ring.
An air stream is typically utilized for drying, classification, and transport of the solid fuel through the pulverizer. The air stream employed is typically a portion of the combustion air referred to as the primary air. The primary air is combustion air first directed through a preheater whereby the combustion air is heated with energy recovered from the flue gas of the furnace. A portion of the primary air is then ducted to the pulverizers. In a bowl mill, the primary air is drawn through beneath the bowl of the bowl mill and up past the roller journal assemblies to collect the pulverized solid fuel. The small particles of solid fuel become entrained in the primary air. The air stream containing the solid fuel then passes through a classifier into the outlet of the pulverizer. After passing through the exhauster, the pulverized fuel can be stored, or more typically, is transported to the furnace by the air stream for direct firing.
The journal loading, which dictates the amount of grinding force that the grinding rolls exert on the coal, to crush solid fuel caught between the roll face of the roller and the grinding ring, has been provided to date either through the use of hydraulic systems or through the use of mechanical springs. One such arrangement of mechanical springs can be found depicted, for example, in U.S. Pat. No. 4,706,900 entitled “Retrofitable Coiled Spring System,” which issued on Nov. 17, 1987 and which is assigned to the same assignee as the present invention. In accord with a showing contained in this U.S. patent, each grinding roll is urged towards the surface of the grinding table by means of an adjustable spring and is rotated about a fixed shaft within the journal assembly and connected to the rotatable grinding roll. To this end, journal bearings allow rotation of the journal assembly relative to the shaft and a spring capable of urging the grinding roll toward the grinding table surface. The spring exerts a predetermined grinding force on the coal disposed on the table when the coal is of a predetermined depth on the table.
Although the journal bearings used in mill of U.S. Pat. No. 4,706,900 have demonstrated to be operative for the purpose for which they have been designed, a need still exists to improve the mill loading and roll life of the bearings. More specifically, the original roll life goal for the journal bearings was 50,000 hours which translated into a roll life of only one or two years using Ni-Hard. However, today mill loading and roll life demand has increased, with projections/demand extending to 82 months (6.8 years) in some instances. Thus, there is a need for longer bearing lives across older, as well as newer, mill lines.
Another factor which deteriorates roll life of the bearings in addition to increased mill loading includes solid fuel dust, such as coal dust, for example, which flows into the journal assembly and contaminates the bearings and lubricant therefor. An interface between the rotatable journal assembly and stationary shaft is exposed to atmospheric conditions and a differential pressure across the journal assembly allows the coal dust, for example, to flow into the journal assembly housing the bearings. The ingress of coal at this interface, which allows the shaft to extend therethrough and rotate with respect to the journal assembly, contaminates the lubricant and journal bearings thus deteriorating the roll life of the journal bearings.
Therefore, there remains a need for a method and apparatus for increasing bearing roll life in a journal assembly, which facilitates increased mill loading and prevents contamination of the bearings, while using as much of the existing journal assembly envelope to reduce costs.